1. Field of Invention
This invention relates to gasifiers as applied to biomass gasification, and more particularly to a small scale high-throughput gasification system using a high efficiency parallel entrained bed pyrolysis unit.
2. Description of Related Art
Fluidized bed gasifier/combustor systems having separate gasifiers and combustor vessels that allow exothermic combustion reactions to be separated from the endothermic gasification reactions, are known in the art. For example, U.S. Pat. No. 3,853,498 to Bailie, describes a process involving separate gasification and combustion zones. In the Bailie process, both zones are conventional fluid-bed reactors. Published wood throughput values for the Bailie process typically do not exceed 120 lbs/ft2-hr. Fluidization would occur typically with inlet gas velocities 1-3 ft/sec to provide good fluidization. Since the Bailie process employs conventional fluid-beds, transfer of circulating sand is by direct flow from fluid-bed to fluid-bed rather than by entrainment and exit out the top of the reaction vessel.
U.S. Pat. No. 4,032,305 to Squires discloses another circulating bed gasifier for coal and coke gasification known as a xe2x80x9cfast fluid-bedxe2x80x9d. The fast fluid-bed can operate in a two-zone configuration of an exothermic combustion zone and an endothermic gasification zone. Squires states that the minimum velocity needed to achieve a circulating fast fluid-bed is a little more than 6 ft/sec with particles having an average diameter of 60 microns. Squires prefers operating with particles no larger than 250 microns.
U.S. Pat. No. 4,828,581 to Feldmann et al. described a novel method of operating a gasifier for production of fuel gas from carbonaceous fuels wherein biomass is gasified at very high wood throughputs in a fluidized bed gasifier operating at low inlet gas velocities. The process relies upon the entrainment of inert solids in a parallel entrained bed pyrolysis process to allow operation at an inlet velocity of as low as 0.5 ft/sec but with a wood throughput from 500 to 4400 lbs/ft2-hr. The lbs/ft2-hr relates to the gasifier diameter by referring to the cross-sectional area of the gasifier.
As illustrated in FIG. 1, the efficiency of the each of these prior systems increases with increasing input of feedstock material. At low inputs the percentage of heat loss increases exponentially, effectively limiting prior systems to inputs of greater than approximately 100 tons per day. If throughput is defined as the ratio of input to cross section, then at high throughputs the curve illustrated in FIG. 1 shifts to the right, becoming even less favorable and requiring a higher overall system input to maintain an acceptable level of efficiency. Accordingly, previous high throughput systems have been limited to operation at feedstock input rates of greater than approximately 100 tons per day.
However, there are many applications wherein in it is impractical to maintain high feedstock input rate on the order of 100 tons per day, such as to provide power small communities or industrial facilities having low power requirements. It would clearly be desirable to operate these systems at a higher throughput because the resulting gasifier unit could be both smaller and cheaper to construct than a conventional low-throughput gasifier of the same capacity. Prior conventional gasifier systems have required a tradeoff between unit cost and efficiency.
Accordingly, there is a need for a biomass gasification system which allows for a relatively low total feedstock input rate while maintaining a high-throughput.
The process system according to this invention relates to production of gas by use of a high throughput combination gasifier and combustor, wherein the exothermic combustion reactions can take place in or near the combustor while the endothermic gasification reactions take place in the gasifier. Heat from the exothermic reaction zone of the combustor is transferred to the endothermic reaction zone of the gasifier by circulation of an inert particulate solid such as sand. In order to increase efficiency by reducing heat loss from the gasifier, the gasifier is concentrically-disposed within the endothermic reaction zone of the combustor.